1. Field of the Invention
The invention relates to counter rotating aircraft gas turbine engines and low pressure turbines with counter rotating fans driven by counter rotating low pressure turbines and, particularly, for such engines and turbines incorporating vanes and having tandem non-interdigitated counter rotating low pressure turbines.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some low pressure turbines have been designed with counter rotating turbines that power counter rotating fans and booster or low pressure compressors. U.S. Pat. Nos. 4,860,537, 5,307,622, and 4,790,133 disclose low pressure counter rotating turbines that power counter rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan. Blade rows or stages of one of the counter rotating turbines turbine rotor are interdigitated with blade rows or stages of another of the counter rotating turbines. No stationary vanes are disposed between the interdigitated rows of blades. A radially outer drum supports blade rows of one of the counter rotating turbines. These blade rows depend radially inwardly from the drum. The drum requires a great deal of mass for structural reasons and is difficult to rotatably support from static frames of the engine.
Advanced gas turbine engines having counter rotating forward and aft fans and counter rotating boosters are being designed. It is desirable to design a counter rotating engine with a peak performance. It has been found that a peak performance can be attained when the forward fan operates at a higher fan pressure ratio and higher rotational speed than the aft fan. This can result in a substantial mis-match in horsepower and rotational speed between the counter rotating rotors. The counter rotating low pressure turbine is required to supply the necessary power to each of the forward and aft fans at the rotational speed of each fan. A conventional counter rotating turbine will operate at peak efficiency when the power split between both shafts is equal and when the rotational speeds are equal and opposite. In such a case, speed and horsepower ratios of the two rotors and turbines are substantially 1. It is highly desirable to have a gas turbine engine with counter rotating low pressure turbines that have different speed and horsepower ratios such as speed and horsepower ratios of 1.2 or more to attain peak fan efficiency. It is also desirable to have light weight counter rotating low pressure turbines that are easily rotatably supported by the engine""s static frames.
An aircraft gas turbine engine includes a high pressure rotor having a high pressure turbine drivingly connected to a high pressure compressor by a high pressure shaft and rotatable about an engine centerline. The engine further includes counter rotatable low pressure inner and outer shaft rotors having low pressure inner and outer shafts, respectively, which are at least in part rotatably disposed co-axial with and radially inwardly of the high pressure rotor. The engine further includes a low pressure turbine section located aft of the high pressure rotor and having a low pressure turbine flowpath and tandem non-interdigitated counter rotating forward and aft low pressure turbines. The low pressure inner shaft rotor includes the aft low pressure turbine and the low pressure outer shaft rotor includes the forward low pressure turbine. The aft low pressure turbine includes first low pressure turbine blade rows disposed across the low pressure turbine flowpath and is drivingly connected to a first fan blade row by the low pressure inner shaft. The forward low pressure turbine includes second low pressure turbine blade rows disposed across the low pressure turbine flowpath and drivingly connected to a second fan blade row by the low pressure outer shaft. The first and second fan blade rows are disposed within a bypass duct radially outwardly bounded by a fan casing.
The first low pressure turbine blade rows are disposed downstream of the second low pressure turbine blade rows along the low pressure turbine flowpath. Rows of non-rotatable low pressure vanes are disposed across the low pressure turbine flowpath between first adjacent pairs of the first low pressure turbine blade rows and between second adjacent pairs of the second low pressure turbine blade rows.
An exemplary embodiment includes an aftmost row of the second low pressure turbine blade rows located upstream of a forwardmost row of the first low pressure turbine blade rows along the low pressure turbine flowpath and has an annular vaneless gap between the aftmost row of the second low pressure turbine blade rows and the forwardmost row of the first low pressure turbine blade rows. A turbine nozzle is disposed axially forward, upstream of, and adjacent to the second low pressure turbine blade rows. The exemplary embodiment has an equal number of the first low pressure turbine blade rows and the second low pressure turbine blade rows.
More particularly, the exemplary embodiment has four of the first low pressure turbine blade rows and four of the second low pressure turbine blade rows. The first low pressure turbine blade rows may be mounted on low pressure first turbine disks of the low pressure inner shaft rotor and the second turbine blade rows may be mounted on low pressure second turbine disks of low pressure outer shaft rotor. Alternatively, the first and second low pressure turbine blade rows may be mounted on drums of the low pressure inner and outer shaft rotors, respectively.
Various configurations of the low pressure turbine may be used. There may be an equal or an unequal number of the first and second low pressure turbine blade rows and there may be three or four or more of each of the first and the second low pressure turbine blade rows.